Conventional electric motors are available in a variety of types depending on their use and the type of electrical current which is used to power them. Motors for use with alternating current include induction motors, synchronous motors, series (universal) motors and repulsion motors. Motors for use with direct current include commutator motors and brushless motors. All of these motors are constructed of three elements--a frame, a stator and a rotor, although the shape and form of these two elements may change. The stator element is generally stationary with respect to the environment and the rotor element generally rotates with respect to the stator. In one well-known type of motor designated a radial gap motor, a cylindrical rotor is attached by bearings to a frame and a stator is disposed around the periphery of the rotor. Alternatively, a cylindrical stator may be attached to the frame and the rotor may be located around the periphery of the stator. Another conventional motor types is an axial-gap motor in which the stator and rotor are disk-shaped.
Both motor elements include magnetic poles which interact to produce the motive forces. Conventionally, a magnetic pole is a electromechanical element which generates a magnetic field of a single polarity. The field which forms the poles may be generated by either permanent magnets or by electromagnets.
As is well-known, in many conventional motors, the magnetic poles are formed by armatures which are electromechanical elements made by winding a coil on a core (or a "core-less" core) made of iron or other materials to produce magnetic pole. The term "armature" in this description is used to describe such elements irrespective of whether the element is part of the rotor or the stator.
The corresponding peripheries of the two motor elements are divided by slots into a plurality of armatures which are energized to form the magnetic poles.
In most known motors, the effective width of the armatures on the rotor is equal--that is the armatures all effectively span the same "center" angle. The center angle is the mechanical or geometrical angle between two radii extending from the axis of rotation along the boundaries of adjacent mechanical elements which form the armatures (if there are air gaps between the elements then the center angle is taken between the radii which extend along the centerlines of the gaps). Similarly, in a conventional motor, the armatures forming the poles of the stator have equal width, although the rotor and stator may have different numbers of poles to prevent lock-up when the motor is starting. "Effective" width means the total width of adjacent elements which have the same magnetic polarity. For example, a plurality of physically adjacent magnetic elements which have the same polarity will be considered as one magnetic pole with a width equal to the the total width of the separate elements.
Magnetic fields are produced on the armatures by windings which are arranged in the slots which define the poles. There are a number of conventional winding techniques which include lap and wave windings. In addition, the windings may be full-pitch or short-pitch windings. As these winding techniques are well-known, they will not be described further in detail. However, in general, known winding techniques use windings wound around the poles in layers. Time varying currents (either alternating currents with various phases or time-varying currents produced by brush commutators or brushless commutation switches) are applied to the windings. The magnetic fields developed by all of the windings at a particular armature add or superimpose to generate a composite or synthesized field at that armature. The timing of the currents is arranged so that the synthesized field rotates with respect to the motor frame and the attraction and repulsion between the rotating field and the rotor magnetic poles provides the motive force.
One problem which arises from conventional winding techniques is that the superposition of the magnetic fields at each armature which produces the rotating field necessarily involves a partial cancellation of portions of the magnetic field developed by each winding layer. Thus, part of the energy which is provided to each winding cancels part of the energy provided to another winding instead of being used to produce a motive force in the rotor. Accordingly, more power must be provided to the motor windings than is theoretically necessary to generate a given power output. In addition, the overlapping winding layers are bulky and make the motor larger than would otherwise be necessary.
More particularly, a conventional radial-gap motor having armatures wound with a full-pitch lap winding and designed to be operated with three-phase alternating current is shown in FIG. 1. In the exemplary motor, the stator 1 is attached to the frame and the rotor 6 is arranged around the periphery of the stator. The rotor is constructed with eight poles 5, which may be formed by permanent magnets or electromagnets. Poles 5 are distributed as alternating north and south poles around the periphery of the rotor. Each of the poles has the same width and spans a center angle of 360.degree./8=45 degrees.
The stator 1 has 24 slots 3 which define 24 mechanical pole pieces 2a. The motor windings 4 which produce the rotating magnetic field of the stator 1 are placed in slots 3 and wound around the pole pieces 2a to form armatures generally designated as numeral 2. A set of windings is associated with each of the three phases (designated as phases .phi.A, .phi.B and .phi.C) of the three-phase alternating current and the windings are connected in a well-known "wye" configuration.
In a conventional fashion, the windings are wound around a set of three adjacent mechanical pole pieces, which, when energized, effectively act as a single electromagnetic pole. For example, winding loop 7 is wound around the pole pieces 8a, 9a and 10a to produce three armatures specifically designated by numerals 8, 9 and 10. These armatures, when energized by the three phase alternating current, generate three poles of the same polarity. Thus, the "effective width" of the resulting pole is equal to the width of the center angle 11 spanned by the pole pieces 8a, 9a and 10a. The winding loops progress around the pole pieces in groups of three with alternating groups of three being wound in opposite senses to produce alternating poles of opposite polarity. As shown in FIG. 1, each slot 3 contains wires from two loops and the windings for each of the three phases, .phi.A, .phi.B, .phi.C, are interleaved or layered as shown in FIG. 1. For clarity, FIG. 1 is a simplified schematic view in which each winding is shown as a plurality of single loops with each loop set into two slots and passing around a set of three poles. In a practical construction, each single loop would consist of many turns around each set of poles.
Such a motor is generally constructed in accordance with the well-known relation, S=MP.tau., where S is the total number of slots, M is the number of phases of the exciting current, P represents the number of poles and .tau. is the winding coil pitch. This relationship gives the number of slots necessary in the stator to achieve the desired winding pitch. In accordance with the above example, M=3, P=8 and .tau.=1, giving S, the number of slots, equal to 24.
The interaction of the rotor and stator poles is shown in FIG. 2 which shows equivalent electrical waveforms taken at a given point in time. In FIG. 2, the motor has been "unwrapped" to produce a linear diagram. The top line of the Figure shows the rotor poles 5. As previously mentioned, each pole spans an mechanical angle of 45 degrees (this angle is equivalent to a mechanical angle of .pi./4 where 2.pi.=360.degree.).
The next three lines of FIG. 2 show the equivalent electrical "poles" generated at a given instant in time by the currents in each of the three phases (as previously mentioned above, three mechanical poles are grouped to form an effective electrical pole). The mechanical "position" of the poles formed by each phase differs because, at any given time, the currents and voltages in each phase differ in electrical phasing.
The last line of FIG. 2 is a graph at the given instant of time of the magnitude of the the magnetic field produced by currents flowing in each of phases .phi.A, .phi.B and .phi.C (indicated by the solid lines marked .phi.A, .phi.B and .phi.C) versus the mechanical position along the stator periphery. In the last line of FIG. 2, a value above the horizontal axis indicates a magnetic "north" pole and a value below the axis represents a magnetic "south" pole. As is well-known, as the currents in each phase change with time, the synthesized field rotates around the stator axis (in the direction to the right in FIG. 2).
At each pole, the magnetic fields are superimposed to generate a synthesized magnetic field which is shown in FIG. 2 versus stator periphery position as a dotted line. Due to the difference in phasing of the three currents, at many positions along the periphery of the stator, the magnetic fields from two phases substantially cancel to produce the synthesized field. For example, at position 12 along the periphery of the rotor, the fields from phases .phi.A and .phi.C substantially cancel. Thus, electrical energy which must be provided to the system to generate the phase .phi.A field cancels the energy used to generate the phase .phi.C field instead of contributing to the motor output. Such cancellation reduces the motor efficiency.
As shown in FIG. 2, in such a conventional 8-pole, 3-phase alternating current motor, the portions of the windings which are not to some extent cancelled and, thus, can effectively generate output torque are only a small portion of the total windings. Consequently, the energy provided to the windings is not used efficiently to obtain a high output torque.
In addition, the windings are overlapped and wound in three layers. Therefore, the physical thickness of the overlapped layers add bulk and weight to the motor and make it difficult to construct small, thin motors such as are used in computer disk drives.
Such a conventional motor is shown in FIG. 3 which is a schematic elevation of a well-known axial-gap motor. FIG. 4 shows a cross-sectional view of this motor taken along lines 4--4 in FIG. 3. As with the radial-gap motor shown in FIG. 1, the axial-gap motor consists of a rotor 6 and a stator assembly 1. Rotor 6 is attached by hub 17 to a shaft 16 which is supported by bearings 14 and 15 mounted in stator 1. A plurality of magnetic pole pieces 5 are mounted in a radial fashion on the inner face of rotor 6. Pole pieces may illustratively be formed from permanent magnets.
Stator 1 also has a plurality of poles formed by windings 4 which are wound on an open form to produce conventional air-cores. The formed winding coils are attached to the inner face of stator 1.
Also, in a fashion similar to the motor shown in FIG. 1, the stator pole pieces of the axial gap motor of FIGS. 3 and 4 is lap wound with three layers of windings 18, 19 and 20. These windings produce the effective pole configurations and magnetic fields shown in FIG. 2 and the motor operates in a similar fashion to the motor of FIG. 1.
The conventional axial-gap motor shown in FIGS. 3 and 4 suffers from the same defects as the similar radial-gap motor in that the portions of the windings which are not to some extent cancelled and, thus, can effectively generate output torque are only a small portion of the total windings. In addition, the three overlapped layers of windings make it difficult to construct a thin, compact motor with high torque.
In order to reduce the size of the motor an alterntive winding technique known as a "short-pitch" winding is conventionally used. In a typical short-pitch winding arrangement, the windings do not extend as far across adjacent slots as in the full-pitch winding technique and, thus, the overlapping or layering of the windings is reduced to some extent. The short-pitch wound motor has fewer layers of windings in each slot and, thus, the bulk and size of the motor is substantially reduced over the conventional full-pitch motor. However, with the same number of turns in the windings, a motor constructed with a short-pitch winding cannot attain the torque/speed characteristics of a motor contructed with a full-pitch winding. To increase the torque of motors using short pitch windings to match that of motors constructed using full-pitch windings, the number of turns in the armature windings must be increased. In order to do this the slots provided in the stator must be made larger and therefore, motor itself must be made larger so that the advantage gained by reducing the overlapping of the windings is largely nullified.
Accordingly, it is an object of the present invention to provide a compact, high-torque electric motor.
It is another object of the present invention to provide a compact, high-torque motor which does not use a full-pitch winding with overlapping layers.
It is still another object of the present invention to provide a compact, high-torque motor which does not use a full-pitch winding, but develops the torque/speed characteristics of a full-pitch winding.
It is a further object of the present invention to provide a compact, high-torque motor in which the magnetic field produced by the armatures is not internally cancelled and so contributes directly to the output torque.
It is a yet another object of the present invention to provide a compact, high-torque motor construction which can be used with motor construction having a variety of pole numbers and phases.
It is a yet another object of the present invention to provide a compact, high-torque motor construction which can be used with alternating current motors, brush commutator motors and brushless D.C. motors.